The presence of microorganisms in waters, especially industrial waters, is a never-ending concern for industrial manufacturers. Examples of industrial waters where microorganisms can interfere with industrial processes include: cooling tower waters, mining process waters, food processing waters, papermaking slurries, pulp and paper mill waters, sugar reprocessing waters, and the like.
In the paper industry, the growth of microorganisms in pulp and paper mill waters can adversely affect finished paper products. Microbial life depends on nutrients, pH and temperature of a particular system. The warm temperatures and rich carbohydrate containing fluids of paper machines and process streams provide ideal growth conditions for a variety of microorganisms. These contaminating microorganisms are capable of causing spoilage of pulp, furnish or chemical additives. The microorganisms cause deposits that break loose and fall into the paper furnish, resulting in quality loss and/or end product defects such as holes and spots. The end result is unsalable paper or paper sold at a lower value. Robertson, The Use of Phase-Contrast Microscopy to Assess and Differentiate the Microbial Population of a Paper Mill, TAPPI Journal, pp. 83 (March 1993).
The presence of microorganisms within industrial water systems results in the formation of deposits of biological origin on industrial machines. These deposits give rise to corrosion, breaks, increased down time, loss of yield, high chemical costs, odors and expensive deposit control programs. In the paper mill industry, slime deposits are reportedly responsible for nearly 70% of all breaks, blockages and pump failures. Safade, Tackling the Slime Problem in a Paper Mill, PTI, p. 280 (September 1988).
Slime may be defined as an "accretion or accumulation caused by certain microorganisms in the presence of pulp fiber, filler, dirt and other materials, mixed in varied proportions, having variable physical characteristics and accumulating at continuously changing rates." Id. In most industrial process waters, especially pulp and paper mill systems, spore forming bacteria and Pseudomonas aeruginosa contribute to slime formation. The latter is most prevalent in paper mill slimes. Fungi is also a contributor of slime formation.
The conventional method of controlling microbial growth is through the use of biocides. Biocides are generally divided into two main groups: oxidizing and non-oxidizing. These biocides act on the microorganisms in one of three ways: either by attacking the cell wall, the cytoplasmic membrane or the cellular constituents. Id. at 282.
While biocides do inhibit microbial growth, economic and environmental concerns require improved methods. A problem with the use of biocides is that high levels of expensive chemicals are needed to control microbial growth. To date, none of the commercially available biocides have exhibited a prolonged biocidal effect. Their effectiveness is rapidly reduced as a result of exposure to physical conditions such as temperature or association with ingredients contained by the system toward which they exhibit an affinity. This results in a restriction or elimination of their biocidal effectiveness.
Therefore, the use of such biocides involves continuous or frequent additions to paper mill systems. Further, these additions must be made at a plurality of points or zones in the system. The costs of the biocides and the labor costs involved are considerable.
Moreover, such chemicals are highly toxic in the quantities known to be required for effective control of microbial populations. As a result, environmental regulations restrict the amount of biocides that can safely be discarded into the environment. Therefore, a need exists for improved methods for controlling the growth of microorganisms in industrial process waters.